Multi-stage sulfur removal system and process for an auxiliary fuel system

ABSTRACT

A process and system for separating a fuel stream containing a low concentration of sulfur compounds from a primary fuel stream is disclosed. The process includes isolating a stage-one permeate stream and a stage-one retentate stream from the primary fuel stream, evaporating the stage-one permeate stream at a vacuum, and isolating a stage-two permeate stream and a stage-two retentate stream from the stage-one permeate stream. The stage-two retentate stream is a fuel stream containing low concentrations of sulfur compounds. The system includes a fuel supply, a stage-one separator for separating a fuel stream into a stage-one permeate stream and a stage-one retentate stream, a stage-two separator, a first supply line connecting a portion of the fuel supply to the stage-one separator, and a second supply line connecting the stage-one separator permeate stream to the stage-two separator. The stage-two separator isolates the stage-one separator permeate stream into a stage-two permeate stream and a stage-two retentate stream.

RELATED APPLICATION

This application claims the benefit of and priority to U.S. ProvisionalApplication Ser. No. 60/649,646, filed Feb. 2, 2005, the contents ofwhich are incorporated by reference herein in its entirety.

BACKGROUND OF THE DISCLOSURE

1. Field of Endeavor

This disclosure relates generally to fuel systems and more particularlyto a system and process for extraction of an auxiliary fuel stream withlow concentrations of sulfur compounds from a primary fuel stream havinghigher concentrations of sulfur compounds.

2. Description of Related Art

Continuing improvements in the performance, cost and durability of fuelcell systems have continued to raise interest in their use as auxiliarypower units (APU), for example, in a vehicle such as a hybrid or fuelcell powered automobile. One limitation to their practical applicationinvolves the logistic fuels or conventional fuels used, for example,diesel and kerosene cuts. These fuels have sulfur content in the rangeof 30 to 3,000 ppm, which either impacts the conversion of these fuelsto hydrogen or the synthetic gas stream, or impacts the performance ofthe fuel cell downstream. One alternative is to require a synthetic “nosulfur” type liquid fuel such as Fischer-Tropsch liquid or gas-to-liquidproducts for the APU, but this forces the need for two separate fuelstreams to be provided for the same vehicle.

Some governmental agencies of the United States for example, havemandated lower levels of sulfur in fuels such as the recent push toward30 to 300 ppm. Even with levels near the lowest limit, the sulfurcontent affects the performance of catalysts in down stream processes,and therefore, preventing effective use of fuel cell systems.Technologies, including membrane based technologies, are being developedfor refinery scale applications to achieve these lower sulfurspecifications. Membrane technologies and specifically pervaporationmembrane technology are commercial technologies that are practiced in anumber of industries for the separation of higher vapor pressurecomponents from a mixture of liquid stream.

It would be desirable to have a simple process that provides the neededseparation selectivity, that could be implemented in a compact,inexpensive system and that would operate with a wide range of primaryfuels from gasoline, kerosene, jet fuel and diesel. It is to these needsthat the present disclosure is directed.

SUMMARY OF THE DISCLOSURE

A system and process for producing an auxiliary fuel stream containinglow concentration of sulfur compounds from a primary fuel streamaccording to an exemplary implementation are disclosed. The systemincludes a first separation stage that isolates a stage-one permeatestream and a stage-one retentate stream from a portion of the primaryfuel stream. A second separation stage isolates a stage-two permeatestream and a stage-two retentate stream from the stage-one permeatestream in which the stage-two retentate stream includes an auxiliaryfuel stream containing low concentrations of sulfur compounds. Thestage-one retentate stream and the stage-two permeate stream are mixedand returned into the primary fuel stream. The result is an auxiliaryfuel stream containing low concentration of sulfur compounds along withthe primary fuel stream.

According to a further exemplary implementation, a process forseparating an auxiliary fuel stream containing low sulfur compounds froma primary fuel stream includes: isolating a stage-one permeate streamand a stage-one retentate stream from the primary fuel stream;evaporating the stage-one permeate stream at a vacuum; isolating astage-two permeate stream and a stage-two retentate stream from thestage-one permeate stream, the stage-two retentate stream comprising afuel stream containing low concentrations of sulfur compounds; andevaporating the stage-two permeate stream at a vacuum for return of thestage-two permeate stream to the primary fuel stream. The stage-tworetentate stream is therefore the auxiliary fuel stream containing lowsulfur compounds.

According to an additional exemplary implementation, a system forseparating a fuel stream containing low concentrations of sulfurcompounds from a primary fuel stream includes: a fuel supply; astage-one separator; a stage-two separator; a first supply lineconnecting the fuel supply to the stage-one separator; a second supplyline connecting the stage-one separator and the stage-two separator. Thefirst supply line conveys a fuel stream from the fuel supply to thestage-one separator. The stage-one separator produces a stage-onepermeate stream and a stage-one retentate stream from the stage-one fuelsupply stream. The second supply line conveys the stage-one permeatestream from the stage-one separator, to the stage-two separator. Thestage-two separator produces a stage-two permeate stream and a stage-tworetentate stream. The stage-two retentate stream produced is fuel streamcontaining low concentrations of sulfur compounds. In this system, thestage-one retentate stream and stage-two permeate are then mixed andreturned to the primary fuel stream.

According to still further exemplary implementation, a system forseparating a fuel stream containing low concentration of sulfurcompounds from a primary fuel stream is disclosed that includes: astage-one separator providing a gaseous stage-one permeate stream and astage-one retentate stream from a primary fuel stream; a first eductorassociated with the stage-one separator, the eductor providing a vacuumand cooling energy to condense the stage-one permeate vapor using thecooled stage-one permeate liquid as the motive fluid for the firsteductor; and a stage-two separator providing a stage-two retentatestream and a gaseous stage-two permeate stream from the condensedstage-one permeate stream, the stage-two retentate stream comprisingfuel stream containing low concentration of sulfur compounds; and asecond eductor associated with the stage-two separator, the eductorusing the cooled stage-one retentate as the motive fluid to provide avacuum for stage-two permeate vapor and condense and mix the stage-twopermeate vapor with the stage-one retentate. The vacuum produced may notonly be generated by the use of eductors with process liquid as motivefluid. Compressed gases such as air or N2 enriched air may be used asthe motive fluid. Alternatively, a motor-driven vacuum pump may beutilized to generate the vacuum.

The features and aspects of the present disclosure will be betterunderstood from the following detailed descriptions, taken inconjunction with the accompanying drawings, all of which are given byillustration only, and are not limitative of the present disclosure.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram of an exemplary embodiment of the processin accordance with the present disclosure.

FIG. 2 is a simplified schematic flow diagram of an embodiment of thesystem in accordance with the present disclosure.

FIG. 3 is a diagram illustrating the results of experimental tests on aperformance model of the system and process shown in FIGS. 1 and 2.

FIG. 4 is a simplified schematic flow diagram of another embodiment ofthe system in accordance with the present disclosure.

FIG. 5 is a partial schematic flow diagram of the stage-one portion ofanother embodiment of the system in which a two step condensationfunction is used on the stage-one permeate.

These and other features and advantages of the present disclosure setforth, in part, in the descriptions which follow and the accompanyingdrawings, wherein preferred embodiments and some exemplaryimplementations of the present disclosure are described and shown, andin part, will become apparent to those skilled in the art uponexamination of the following detailed description taken in conjunctionwith the accompanying drawings and the appended claims.

DEFINITIONS

In this discussion, the terms used are used according to their plainmeaning as intended by a person skilled in the art. In the describedembodiments below, the terms below are intended to be used as follows.

The term “hydrocarbon” is generally used to describe a organic compoundprimarily composed of hydrogen and carbon atoms of various lengths andstructures but may also contain non-carbon atoms (such as oxygen,sulfur, or nitrogen).

The term “straight chain hydrocarbon” implies that the compound is aparaffin or isoparaffin type hydrocarbon without ring structures.

The term “pervaporation” means separation of mixtures of liquids bypartial vaporation through a non-porous membrane.

The term “naphthenes or naphthenic compounds” are hydrocarbons with oneor more rings of carbon atoms with only single bonds.

The term “aromatic compound” is a hydrocarbon containing one or morerings of carbon with double bonds within these rings.

The term “heterocyclics” are aromatic or naphthenic compounds thatcontain atoms in addition to carbon and hydrogen such as sulfur,nitrogen or oxygen. Heterocyclics are usually polar in nature.

The term “polar” or “ionic”, as referred to membranes, indicatesmembranes that contain ionic bonds. Similarly, the term “non-polar” or“non-ionic” are membranes that do not contain ionic bonds. If themembrane is polar in nature or coated with a polar material such asNafion® (fluorinated polymer with branched sulfonic acid groups), thecompounds with polar species will tend to be selectively transferredthrough the membrane. Non-polar membranes, such as cellulose triacetate,will tend to be selective to non-polar compounds and selectivelydecrease the transfer rate of ionic compounds.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present disclosure relates to a pervaporation membrane process andsystem for separating a slip stream or auxiliary fuel stream from aprimary fuel stream.

It should be appreciated that for simplicity and clarity ofillustration, elements shown in the Figures and discussed below have notnecessarily been drawn to scale. For example, the dimensions of some ofthe elements are exaggerated relative to each other for clarity.Further, where considered appropriate, reference numerals have beenrepeated among the Figures to indicate corresponding elements.

FIG. 1 shows a simplified schematic representation of the overallprocess according to this disclosure, indicating two separation stagesand the selectivity for the various elements of the fuel feed mixture.The process 1 comprises flowing a portion of a primary fuel stream 2through a stage-one separator 11 and then flowing a separated portionthrough a stage-two separator 16. The primary fuel 2 can be anycommercially available transportation or logistics fuel (such as, butnot limited to, reformulated gasoline, kerosene and diesel commonlyavailable at interstate refueling stations, jet fuels, aviationkerosenes (A-1) or military specification fuels like JP-8, NATO F-76,etc.) These fuels consist of a wide mix of hydrocarbon components, whichtypically have been separated from a bulk fuel source through a vaporfractionation process during refining. As a result, each mixture has arange of compounds that each vaporize over a defined temperature band.In general, gasoline contains lighter components than kerosene which inturn is lighter than diesel fuels. Within each volume of fuel of aspecific type there exists a range of compounds with various vaporpressures and polar and non-polar characteristics.

The primary fuel feed 2 is in particular composed of a mixture ofcomponents that can, in particular, consist of a mixture of hydrocarbonsthat can be subdivided into five primary groupings. The first group isthe heavy sulfur compounds (H Sulfur) 22 which are the multi-ringed andmulti-branched hydrocarbons with relatively high boiling points. Forexample, these include the di-benzothiophenes with boiling pointsgreater than 300° C. Di-benzothiophenes contain at least three ringstructures and are aromatic compounds. The second group is the lightsulfur compounds (L Sulfur) 24, which are the simpler compoundscontaining sulfur with relatively lower boiling points and less thanthree ring structures. For example these include the various asmercaptans, thiophenes, and benzothiophenes with boiling points lessthan 300° C., and more preferably below 225° C. The third group is theheavy hydrocarbons (H HC) 21, and the fourth is the light hydrocarbons(L HC) 25, neither of which contains sulfur atoms, which were includedin the first two groups. The fifth group is the additives 23, which arespecific to each type of fuel depending on the manufacture, themanufacturer, and other criteria. For example, military JP-8 fuel issimilar to commercial aviation fuel except that three additives arerequired in the specification. For JP-8 these are fuel system icinginhibitors (MIL-DTL-85470) di-ethylene glycol monomethlylether(di-EGME), corrosion inhibitor/lubricity improvers (MIL-PRF-25017 andParadyne 655) and electrical conductivity/static dissipater (Stadis®450and MIL-S-53021). These specifications are set forth in the “GUIDE FORFIELD BLENDING OF ADDITIVES OR FOR WINTERIZING GROUND FUELS”,AMSTA-TR-D/210, U.S. Army Tank-Automotive Research, Development andEngineering Center (TARDEC), Warren, Mich. 48397-5000, October 1999,incorporated herein by reference in its entirety.

In the process 1 shown in FIG. 1, primary fuel feed 2 is separated intoan auxiliary fuel stream 3 and a return primary fuel stream 4 in amulti-stage process 1 consisting of a stage-one separation process 11and a stage-two separation process 16. The stage-one separation process11 supports the transfer through a membrane 12 of the lighter compoundsconsisting of light sulfurs 24 and light hydrocarbons 25, which exit asthe stage-one permeate 13. The other compounds 21, 22, and 23 areretained in the retentate stream 14 also resulting from the stage-oneseparation process 11. The stage-one permeate 13 is passed to thestage-two separation process 16. In the stage-two-separation processlight sulfur compounds 24 are selectively transferred through membrane17 along with some of the light compounds 26 as the stage-two permeate19 while most of the lighter hydrocarbons 28 are retained in thestage-two retentate stream 18. The stage-two permeate 19 is mixed withthe stage-one retentate 14 and returned to the primary application ortank as the return primary fuel stream 4. The stage-two retentate 18becomes the auxiliary fuel supply 3.

Although the stage-one 11 and the stage-two 16 processes are selectiveprocesses, their selectivity is not necessarily absolute. By controllingthe temperature of the feed streams 2 and 13 and the vacuum of thepermeate streams 13 and 19, the quantity of compounds actuallytransferred through the membranes 12 and 17 can be controlled.

The motive force for permeation is the partial pressure difference (PPD)of the compounds passing through the membranes in each stage. Thepermeation rate is enhanced as the temperature increases. Temperaturelimitation of the membrane materials may suggest a lower operatingpoint, but applying vacuum in the permeate side would give higher PPDmotive force to compensate for the lower temperature. As the temperatureof the feed stream is increased, the mobility of the compounds isincreased and permeability of the membrane is increased. Both parametersincrease the rate of transfer of compounds through the membrane. Thetemperature of the membrane also increases the vapor pressure ofcompounds exposed to the permeate side of the membrane.

In a process 1 using pervaporation membranes, a vacuum in a range of 1to 500 torr, and more preferably between 100 torr to about 200 torr, maybe applied to the permeate side and the compounds with the highest vaporpressure are evaporated from the surface of the membrane as a vapor. Theoptimal temperature range depends on the level of vacuum, the type ofprimary fuel, and the membrane material that is utilized. For example,where the primary fuel is an aviation kerosene and the light hydrocarboncut from the stage-one process 11 is in the gasoline boiling range, anda vacuum is applied across the membrane, then the temperature rangeshould be between about 100° C. and about 200° C., with a preferablerange between about 120° C. to 135° C. when a vacuum of 100-200 torr isapplied. If, on the other hand, the primary fuel is a diesel cut, thenthe temperature may be in the 150° C. to 300° C. range. The evaporationof the compounds with the highest vapor pressure creates a concentrationgradient across the membrane, which promotes the enhanced transfer ofthese compounds from the feed stream. Compounds with low vapor pressuresdo not evaporate and therefore, a concentration gradient is notestablished for these compounds and additional transfers do not occur.Other parameters such as the membrane surface area, membrane thickness,and internal fluid phase mixing on the retentate side of the membranealso contribute to the quantity of compounds transferred. Increases inmembrane surface area provide enhanced transfer because of the largerarea for transfer and the larger evaporative transfer area on thepermeate side. Thinner membranes have lower resistance for transport andsupport higher transfer rates because the concentration gradient that isestablished functions over a shorter distance.

The permeate flux is inversely proportional to the membrane thicknessand hence a thinner membrane is preferred if it can survive at theoperating pressure and temperature. Internal fluid phase mixing enhancestransfer rates by ensuring mixing of the various compounds in the feedstream and maintaining the highest concentration of high vapor pressurecompounds possible at the feed surface of the membrane. The masstransfer resistance in the bulk flow on the retentate side may reducethe effective partial pressure at the membrane surface. Turbulent flowor high flow velocities can improve mixing and increase flux. Adjustingthese characteristics of the membranes in stage-one separators andstage-two separators help ensure that all of the light sulfur compounds24 which are transferred in stage one 11 are again transferred in stagetwo 16. By operating at low enough temperatures where higher boilingcompounds do not vaporize, the amount of heavy aromatics or multi-ringsulfur compounds such as the di-benzothiophenes going through into thepermeate can be reduced and minimized.

The stage-one separation process 11 and stage-two separation process 16in accordance with the present disclosure can be carried out utilizing anumber of techniques ranging from flash vaporization separation (FVS),use of filtration membranes such as reverse osmosis (ROM),nano-filtration membranes (NFM) and ultra-filtration membranes (UFM) topervaporation membranes such as non-polar pervaporation membranes (NPVM)and polar pervaporation membranes (PPVM). Proper selection of thespecific separation technique for each stage is dependent on thespecific primary fuel and the distribution of sulfur compounds withinthe mixture. The specific additives 23 and sulfur compounds in theprimary fuel feed 2 can also influence the selection of which separationtechnique is optimum.

The separation techniques suitable in the separation processes hereindisclosed, and their selectivity with respect to the compounds ofinterest in all embodiments of this process can be identified byconsideration of three characteristics. The first is physical size ofthe pores, the second is selectivity by polar or ionic characteristic ofthe compounds such as sulfur compounds, and the third is selectivity byboiling point or vapor pressure. One exemplary embodiment of the presentdisclosure involves a first step of separating a low boiling fractionpermeate for processing through a sulfur-selective membrane in a secondstep and collecting the light retentate as the clean product for downstream processes such as an auxiliary fuel source. The high boilingfraction retentate from the first step and the permeate from the secondstep are combined and sent back as the residual fuel.

Simple filtration processes such as ROM, NFM and UFM provide separationprimarily by physical size and structure. ROM separation devices haveeffective pores sizes below 0.0014 micrometers, while NFM devices havepores form 0.0008 and 0.005 micrometers and UFM devices have pores from0.0025 to 0.1 micrometers. These relate to compound selectivity bymolecular weight to some degree. For example ROM is good for compoundsbelow 300 gm/mole, NFM is good for between 160 to 10,000 gm/mole and UFMis good for between 2,000 and 200,000 gm/mole. In addition to mass, thethree-dimensional shape of the molecules also impacts selectivity withcomplex multi-ring structures having greater steric hindrances thanlinear straight chain compounds.

Separation technologies such as flash vaporizers and pervaporationmembranes are selective to the boiling point, or vapor pressure, of thecompounds. For example, in a flash vaporizer, a liquid feed ispressurized and heated. This heated and pressurized fluid is then passedthrough an orifice creating a rapid pressure reduction and evaporationof some of the low-boiling point compounds within the mixture takesplace. Heavier compounds with higher boiling points remain as a liquid.Thus the fuel feed will then be a two phase mixture. This two phase(gas-liquid) mixture is then separated into a gas stream and a liquidstream. The gas stream is then condensed to complete the separationprocess. Utilization of a pervaporation membrane is similar, except, itcombines the separation processes of membranes and vaporization.

In one embodiment of a system in accordance with the present disclosure,a flash vaporizer equipped with a fractional distillation unit is usedas the first stage, and polar pervaporation membrane is used in thesecond separation stage. In another embodiment, polar pervaporationmembranes are used to perform both the first and second stageseparations. These PPVMs combine the physical filtration and polarselectivity to preferentially isolate polar compounds of small size, andthen apply additional selectivity because the transferred compounds thatare vaporized from the permeate side of the membrane. If a compoundphysically can transfer through the membrane, but its vapor pressure istoo low at the operating temperature to be vaporized, it will not becarried over to the permeate phase. Therefore, a PPVM separatoroperating at a defined temperature can demonstrate selectivity to bothlight hydrocarbons and light sulfur compounds.

In another embodiment of the system in accordance with the presentdisclosure, the stage-two separator has a greater selectivity to polarmolecules than the stage-one separator. This characteristic helps ensurethat all polar sulfur compounds and other ionic compounds, whichtransfer through the first membrane, are returned to the primary fuel byway of the second stage permeate flow.

An exemplary embodiment of the process is designed to provide alow-sulfur, clean fuel stream usable in an auxiliary power unit (APU)onboard a vehicle by extracting a fraction of the fuel being sent to theprimary application, such as to drive the engine or primary generator.An important parameter in the evaluation of the efficiency of theprocess is the ability to extract a low sulfur fraction of the fuelmixture while leaving behind substantially all of the fuel's performanceadditives and sulfur compounds in the return primary fuel stream.

FIG. 2 shows an embodiment of the process 1 of FIG. 1 implemented in asystem 60. In the system 60 primary fuel from the tank 61 is pressurizedby pump 63 in a primary fuel feed stream 32. The primary fuel feedstream 32 is then heated by the process recuperative heat exchanger 53to a primary fuel first hot feed stream 321. The primary fuel first hotfeed stream 321 is then heated by the hot engine fluid 50 in the heatexchanger 52 to a primary fuel second hot feed stream 322. This secondhot feed stream 322 is then feed to the stage-one separator 31 in whichit is separated into the stage-one retentate stream 37 and stage-onepermeate stream 34. The stage-one permeate stream 34 is then conveyedinto eductor 35 which is driven by the eductor's motive fluid 38. In theeductor 35, the stage-one permeate vapor stream 34 is put at vacuum,where the vacuum is due to the feed stream of eductor motive fluid 38.Exiting the eductor 35 is the combined liquid stream 36, which is amixture of the stage-one permeate stream 34 and the eductor motive fluid38. This combined liquid stream 36 flows to cooling heat exchanger 57where it is cooled. The weight ratio of cool motive liquid 38 tostage-one permeate vapor 34 is kept suitably high such that thecondensation heat only raises the temperature of the combined liquidstream 36 only slightly. The condensation of vapor into liquid resultsin lower volume and thus vacuum is created upstream. Also the use of aneductor with high velocity motive fluid also creates vacuum.

The hot liquid stream 36 is cooled in heat exchanger 57 into a coolliquid stream 360 which is increased in pressure by pump 137 and splitsinto motive stream 38 sent to eductor 35 and stage-two feed stream 42.Cooling unit 55 is heat rejection element consisting of an air cooled orliquid cooled radiator or other appropriate mechanism such as thevehicle's air conditioning system through which excess heat can beremoved form the system. Cooling fluid streams 362 and 363 are coolingloops conceptually illustrated to transfer heat from the three coolingheat exchangers 56, 57, and 58 to the cooling unit 55. Although a seriesflow configuration is shown in the disclosure, it is not limited by thisillustration and any combinations of series or parallel flows andintegrated or dedicated cooling units are feasible and covered in thisdisclosure. Recuperative heat exchanger 53 transfers the heat fromreturn fuel stream 46 to feed stream 32. To ensure that all the vapor inreturn stream 46 is condensed cooling heat exchanger 56 is included.

A portion of the liquid stream 361 is passed onto the stage-twoseparator 41, as stage-two feed flow 42. In the embodiment shown in FIG.2, only a small part of the liquid stream 361 is passed onto thestage-two separator 41, while the most part flows to the eductor aseductor motive fluid 38. The use of liquid stream 361 as eductor motivefluid 38 eliminates the necessity of using a separate working fluid withits gas-liquid separators and dual downstream cooling heat exchangers.In steady state operations, the composition of the gas-liquid stream 36is primarily the light hydrocarbon and light sulfur compounds asdiscussed more in details below. In steady-state operation thecomposition of fluids 34, 36, 38 and 361 are the same that support thedirect condensation of gas-liquid stream 36 after it exits the eductor35.

The stage-two feed 42 is passed to re-heat heat exchanger 51 and becomesa hot stage-two feed 421 that enters stage-two separator 41. The feedstream of the hot stage-two feed 421 is separated into a stage-twopermeate stream 44 and a stage-two retentate stream 47. The stage-twofeed 42 may, in some alternative embodiments, first be heated throughrecuperative heat exchangers (not shown) interfaced with the outletstreams from stage two, either stage-two permeate 44 or stage-tworetentate 47 prior to entering re-heat exchanger 51. The sulfurselectivity of the membrane in stage-two separator 41 supports thetransfer of a majority of the sulfur compounds in the stage-two permeatestream 44 while maintaining some of the light hydrocarbon compounds asthe stage-two retentate stream 47 which is cooled and stored in tank 62as the auxiliary fuel supply. The better the selectivity of thestage-two membrane the more light hydrocarbons are retained in theretentate stream 47. The stage-two permeate stream 44 is put at a vacuumdue to the action of a motive fluid conveyed into an eductor 45. In thisembodiment, the motive fluid for eductor 45 is the stage-one retentatestream 37. In another alternative, stage-two feed 42 can be heated withone or more regenerative heat exchangers interfacing with the stage-oneretentate stream 37.

Use of the stage-one retentate stream 37 to drive the eductor 45minimizes the complexity of the process. Exiting from the eductor 45 isthe returning fluid 46 passing through the recuperative heat exchanger53 to improve process thermal efficiency. The fluid is finally passedthrough cooling heat exchanger 56 before returning to the tank 61 toensure the returning fluid is at or near ambient temperature.

One or both of the permeate streams mal also be cooled in one or moreseparate cooling heat exchangers to enhance vacuum creation. In otherembodiments, the stage-one retentate stream 37 is cooled prior tofunctioning as the motive fluid for eductor 45. For example the eductor45 can be positioned downstream of recuperative heat exchanger 53 suchthat retentate stream 37 flows through heat exchanger 53 prior toentering eductor 45 as the motive fluid. In this embodiment, the motivefluid for eductor 45 is the stage-one cooled retentate stream 37. Inanother embodiment, one or both the permeate streams are also cooled inone or more separate cooling heat exchangers to enhance vacuum creation.

In other embodiments, exiting from the eductor 45, the sulfur compoundstransferred as permeate in the stage-two separator can be mixed with theretentate from stage one and returned to the primary engine or theprimary fuel tank 61.

The system disclosed above can be used to perform a multi-stageseparation process on board a vehicle to extract a low sulfur slipstream, in the stage-two retentate 47 of fuel compounds which can beused for auxiliary power units, while designing the separation stagessuch that performance enhancing additives in the primary fuel 32 areretained in the stage-one retentate 37 or returned to the stage-oneretentate by way of the stage-two permeate 44. In prior art andconventional approaches a slip stream of the primary fuel is removed asthe auxiliary fuel stream and then treated to completely destroy, adsorbor absorb the sulfur compounds.

This approach allows the extraction of a low-sulfur, clean fuel streamby way of a multi-stage process returning the sulfur compounds in theresidue to the primary fuel instead of adsorbing or absorbing them on toa throw away filter or a bed of solids and destroying the sulfurcompounds in the regeneration cycle. This approach also enhances theperformance of third stage adsorbents or absorbents, because the otherpolar compounds in the fuel are also reduced in concentration by thefirst two stages. Adsorbent storage capacity with the most commonly usedfuels was found to be one-tenth their capacity in comparison tosurrogate fuels which have been doped with specific sulfur compounds.This drop in capacity is due to other compounds such as additives orheavy aromatics, which are also polar in nature occupying active sitesfor adsorption.

One embodiment of the system focuses on using the retentate from stageone 37 as the motive fluid for the stage-two eductor 45 that providesthe vacuum on the stage-two permeate side. This retentate stream can becooled to enhance the performance of the eductor creating greater vacuumon the stage-two permeate side. Similarly, the evaporated permeatestream 44 can also be cooled and condensed prior to entering the eductor45 to further enhance performance. Also the stage-one retentate 37 canbe cooled prior to entering the eductor to further enhance vacuumperformance. Cooling of these fluids around the eductor can be throughdirect cooling or recuperative cooling in combination with preheatingfeed streams to the stage-one or stage-two membrane processes.

One embodiment of the system focuses on using the stage-one permeatestream 34 after condensation and pressurization as the motive fluid 38for the stage-one eductor 35.

One embodiment of the system focuses on the returning fluid 46 passingthrough the recuperative heat exchanger 53 to improve process thermalefficiency. The fluid is then finally passed through cooling heatexchanger 56 before returning to the tank 61 to ensure the returningfluid is at or near ambient temperature.

One embodiment of the system includes the use of the thermal energy fromthe primary engine application to provide the peak thermal energy needfor effective operation of the stage-one and stage-two separatormodules. This is illustrated in FIG. 2 with engine 50 and heatexchangers 52 and 53.

Another embodiment of the system process includes further integrationwith the primary vehicle. In this embodiment the cooling loop source 55is integrated with the vehicle's cabin air conditioning system tominimize component redundancy. Depending on the primary fuel type (suchas JP-8, diesel, or gasoline) the vapor pressure of the light compoundsin stream 34 and the pressure of the mixed stream 36 the temperatureneeded to fully condense the mixed stream 36 into liquid stream 361 maybe less than ambient temperatures. In this situation the cooling loopelement 55 may be integrated into the vehicle's air conditioning loopwhich has been designed to cool the cabin air. Alternatively, adedicated cooling unit capable of sub-ambient temperatures may be usedas cooling element 55.

The system and process represented in FIG. 1 and FIG. 2 have beenmodeled to assess the ability to effectively provide a clean, low-sulfurauxiliary fuel supply. FIG. 3 shows a diagram reporting the results ofthe performance of such a model. The model was developed assuming a feedfuel stream similar to JP-8 military fuels with 500 ppm of sulfur byweight. A typical JP-8 fuel has a boiling point range from approximately140° to 300° C. and is composed of multiple compounds: 57% paraffins,20% cycloparaffins, 20% aromatics, and 3% misc.

The data from the model was plotted on the chart with the y-axisindicating the sulfur content of the fuels. The x-axis is a measure ofthe type of sulfur compounds in the feed as represented by the relativesulfur concentration in the lighter fraction in comparison to the totalfeed. Therefore, a value of 100% indicates that the sulfur concentrationis uniform across the fuel's boiling point range. Detailed sulfuranalysis of several samples of Jet-A fuel with bulk sulfurconcentrations in the range of 1500 ppm to 3000 ppm has have indicatedthat a majority of the sulfur compounds are relatively heavy consistingof di-benzothiophenes type compounds. When a 30% light fraction isobtained approximately 70% reduction in the sulfur concentration isachieved, indicating a bulk fuel with 1500 ppm sulfur will produce alight fraction with 450 to 500 ppm sulfur. This indicates the “relativesulfur in light fraction” is approximately 30 to 35% for typical Jet-Afuel, obtained from commercial sources in Long Beach, Calif. in 2005.Modifications in up-stream refinery process may change thischaracteristic.

The feed concentration of the fuel is represented as the gray line 70and would be the concentration of the stream 32 of FIG. 2. The stage-oneseparator was modeled as a non-selective separation and 30% of the bulkfuel was extracted in the stage-one permeate 34 of FIG. 2. Since theseparation was not selective the line with open circles 71 representsthe relative sulfur in the light fraction which in this case is 30% ofthe bulk fuel.

The stage-two separation extracted 40% of the feed stream 42 as permeate44 and achieved a selectivity which removed 98% of the sulfur in thefeed. This data is consistent with published performance data from W. R.Grace's PPVM modules for Benzo-thiophenes and lighter sulfur compoundsin an intermediate cut naphtha fuel stream (Zhao X., Krishnaiah G., andCartwright T.; Membrane Separation for Clean Fuels; PTQ Summer 2004) thecontent of which is incorporated herein by reference in its entirety.

The performance of the stage-two separation is illustrated as the opensquare line 72, which indicates that sulfur levels of under 10 ppm areachievable if the relative sulfur in the light fraction is less than60%. If the required sulfur level in the auxiliary fuel is under 10 ppmor the relative sulfur concentration greater than 60%, a third stage maybe needed. In this example the stage-two retentate 47 was furthertreated with a liquid-phase sulfur adsorbent or absorbent in a thirdstage adsorbent or absorbent module, which achieved a 95% removalperformance. The resulting concentration of sulfur in the product streamis represented by the open triangle line 73 that illustratesconcentration levels less than 2 ppm across the complete range ofrelative sulfur levels in the light fraction.

Further embodiments of the process and system can include the additionof this third stage adsorbent or absorbent module to polish the finalauxiliary fuel stream. FIG. 4 shows an exemplary implementation of suchembodiments. With reference to FIG. 4, the system is identical to thatof FIG. 2 except that a sorbent-type polishing filter bed 147 is addedto the process system in the stage-two retentate stream 47. If the downstream applications for the slip stream require very low sulfurconcentrations then the polishing filter bed 147 is added. Sulfurcompounds in the stage-two retentate are trapped in the sorbent-type bedsuch that the stream 148 leaving the bed 147 has very low sulfurconcentrations similar to the model results presented as line 73 of FIG.3.

The polishing filter bed 147 may be an adsorbent similar to high surfacearea carbon or carbon with specific surface modifications tailored toadsorbing the polar sulfur compounds. The bed 147 may be an absorbent orreagent designed to react with the sulfur atom in the compoundsresulting in a non-soluble sulfur inorganic salt that is trapped in thebed.

In summary, the present disclosure relates to a multi-stage process forthe separation of a slip stream of fuel with low-sulfur concentrationfrom a primary multi-component fuel stream. The primary multi-componentfuel is fed to the first stage in which a slip stream is isolatedcontaining lighter components and the heavier components are retained inthe retentate stream. The lighter component slip stream is furtherprocessed by the second-stage membrane in which the sulfur components ofthe slip stream are selectively removed form the feed as second-stagepermeate and a second-stage retentate is recovered as the low-sulfurauxiliary fuel supply.

In another aspect, the disclosure relates to a multi-stage process inwhich one or more of the stages are a selective membrane, and mostspecifically a polar or ionic, pervaporation membrane which has beendesigned to have selectivity for lighter sulfur compounds. Polymericmembranes such as Nafion (DuPont), specially treated polyimides such asS-Brane (W. R. Grace) are examples of sulfur selective polarpervaporation membranes.

In another aspect, the disclosure relates to a multi-stage process inwhich one or more of the stages are a non-selective, high Flux membrane.As with other preferred embodiments the stage-one permeate 34 iscondensed and fed to a second-stage membrane process 41. The high fluxmembrane is selected when the relative sulfur in the light fraction isless than 100%, as has been validated for some Jet-A type fuels.

As with any evaporation, distillation, or pervaporation process thespecies evaporating from the bulk fluid is driven by the vapor pressureof the individual species at the liquid gas phase interface establishedwithin the membrane. The vapor pressures of the individual species arestrongly dependent on the temperature, but all species have some finitevapor pressure, and therefore, as the light fractions evaporate traceamounts of the heavier species also evaporate. Depending on the specificsulfur compound species in the bulk fuel and the trace levels of thesespecies in the stage-one permeate 34 further refinements to thestage-one process may be needed to achieve an overall effective system.Another preferred embodiment of the system divides the stage-one processinto a stage-1A and stage-1B process. Both stages are pervaporation,flux membrane, distillation, and/or evaporation type processes. Thepermeate stream for the stage-1A is condensed, reheated and sent as thefeed stream to the stage-1B process. The permeate stream for thestage-1B is condensed and sent to the stage-two process as previouslydescribed. This secondary stage-one process helps to enhance theselectivity through dual filtering of the trace species. The retentatestreams from both the stage-1A and stage-1B are returned to the primaryfuel tank.

Another exemplary embodiment of the system in accordance with thepresent disclosure addresses the issue of trace amounts of heavierspecies in the stage-one permeate 34. In this embodiment a two-stepcondensing function is integrated into the process after the stage-onepermeate stream 34 is created. FIG. 5 illustrates one preferredembodiment of this condensing function configuration applied to thesystems shown in FIGS. 2 and 4. Here, like numbers are used to designatelike elements previously discussed with reference to FIGS. 2 and 4.

The stage-one permeate 34 is passed through a partial condenser 370 inwhich the heaviest species are converted into liquid creating a twophase flow 381. The two phase flow 381 is passed into a gas-liquidseparator 372 creating a liquid stream 384 and a vapor stream 382. Theliquid stream 384 is returned to the primary tank 61 using a pump 373and a return connection 385. The vapor stream 382 is condensed in a heatexchanger 371 and passes to the eductor 35 by way of connection 383. Theprocess down stream of eductor 35 is similar to other embodiments of thesystem described above with reference to FIG. 2.

The multi-stage process in accordance with the present disclosure may beconfigured to have the pressurized retentate stream for the first stageflow through the motive side of an eductor to create a vacuum on thepermeate side of the second stage. In this configuration all permeatecompounds are returned to the primary fuel stream and the process isfurther simplified by eliminating the requirement for vapor liquidseparation hardware. Enhancements of this process and system can includethe cooling and/or condensation of the feed streams into the eductor 45to improve the vacuum performance creating a lower pressure for thestage-two permeate stream 44. This cooling function can be direct,through the integration of a heat exchanger connected to the coolingsystem 55, or can be achieved with recuperative heat exchangersinterfaced with other streams such as stage-two feed stream 42 prior toheating by heat exchanger 51, or with other streams such as stage-onefeed stream 321 prior to heating by heat exchanger 52. Other thermalrecuperation aspects and flow configurations are feasible to enhanceprocess energy efficiency and the embodiments of this process are notlimited by the configurations that have been illustrated for simplicity.

A multi-stage process in accordance with the disclosure may beconfigured such that the condensed permeate of the first stage flowsthrough the motive side of an eductor to create a vacuum on the permeateside of the first stage. In this configuration the process is furthersimplified by eliminating the requirement for vapor liquid separationhardware. In yet another aspect, the disclosure relates to a multi-stageprocess in which the thermal energy in the mixture of the first stageretentate and the second stage permeate is recuperated and transferredto the first stage feed to minimize thermal energy requirements.

A multi-stage process in accordance with the disclosure may beconfigured such that the peak thermal energy is provided to the stagesthrough integration with the reject heat from the primary engine in avehicle. The thermal cooling energy may be provided to the stagesthrough integration with the vehicle's vapor compression systemtypically used to condition the air within the vehicle's cabin.

In yet another aspect, the multi-stage process may be configured suchthat membrane systems are used to extract a low-sulfur slip stream whichis later used in a fuel cell or hydrogen generation system. For example,the initial stage membrane systems may also be integrated with a lastpolishing stage consisting of absorbent or adsorbent materials designedto extract the sulfur compounds to levels below 10 ppm concentrations.

The following modeled examples 1 through 4 are provided to describe thedisclosure in further detail. These modeled examples, which set forth apreferred mode presently contemplated for carrying out the disclosure,are intended to illustrate and not to limit the disclosure.

EXAMPLE 1 Auxiliary Clean Fuel Stream for a 2-10 kW APU

Trucks and HumVee type vehicles use varying quantities of diesel orlogistic fuel (JP-8) depending on the speed, load being carried andother parameters. Usually main engine needs to operate even duringnon-drive time to supply heating or A/C or communication power needs. Inorder to improve the overall fuel efficiency and to reduce the timesmain engine need to be operated, the heating and air-conditioning andnight-watch communication power may be supplied by a fuel cell operatingon hydrogen that will be generated using a low-sulfur clean fuel(approximately, 5 to 15% of the bulk fuel) separated from the primaryfuel.

Making reference also to the diagrams illustrated in FIGS. 1, 2 and 4,primary fuel from fuel tank 61 having a sulfur content of 500 ppm ispumped to 100 psig and sent to heat exchangers 53 and 52 where itstemperature is raised to 250° C., and then fractionated, separated orpartially vaporized in unit 31 by letting down to ambient pressure. Thevapor stream or permeate stream 34 is condensed (about 25% of feed byvolume) by mixing in an eductor unit 35 with condensed recycle stream(38). The condensed light hydrocarbon stream contains about 400 ppm byvolume of sulfur but is free of additives contains only trace amounts ofhigher boiling hydrocarbon components. Then the condensed light stream42 is sent to an exchanger 51 and to membrane separator 41, where it issplit into a liquid retentate stream 47 and a vapor permeate stream 44,which is combined with the heavy stream 37 from unit 31 and returnedback to primary fuel tank as stream (45) containing about 550 ppm S andall of the additives. The stream 47 contains less than 9 ppm sulfur andis the clean auxiliary fuel that is used in APU reformers (not shownhere) for making syngas or hydrogen for use in fuel cells that produceelectric power.

EXAMPLE 2 Auxiliary Fuel Containing 1 ppm or Less

Where some reformers may need auxiliary fuels that contain 1 ppm or lesssulfur, then another separation (like unit 147) is added as describedbelow and illustrated in FIG. 4. The retentate stream 47 is passedthrough separator 147, which is a polishing stage that absorbs andretains the remaining sulfur species. Ultraclean retentate with lessthan 1 ppm sulfur is generated as stream 148 and cooled if necessary inunit 58 and stored in the clean auxiliary fuel tank or used by the APUreformer system (not shown here).

EXAMPLE 3 Auxiliary Clean Fuel for APUs in Aircrafts

Currently commercial and civilian aircrafts have APUs that are generally50 to 250 kW in size that use JP-8 fuel in combustion process at lowefficiencies and producing pollutants even when they are on the ground.These APUs can be converted to low or non-polluting hydrogen based fuelcells as discussed in Example 1, using auxiliary fuel generated from thebulk fuel in the aircraft.

EXAMPLE 4 Auxiliary Clean Fuel for APUs on Ships

Naval ships and merchant marine vessels can also be equipped with theauxiliary fuel production and reformers and fuel cells producing cleanpower while these ships are docked at port, as taught in Example 1 butat larger capacities 250 to 750 kW scales.

The following examples 5 through 7 represent the results of experimentsactually performed rather than results of modeling.

EXAMPLE 5 Flash Vaporizer-Fractional Distillation as the Stage-OneSeparator and Polar Pervaporation Membrane as Stage-Two Separator with20% Cuts in Both Stages

Jet Fuel (JetA) sample containing 1530 ppm sulfur was subjected tofractional distillation and the distillates were found to contain thefollowing sulfur levels. The Jet fuel sample was analyzed using a gaschromatograph equipped with atomic emissions detector (AED).

TABLE A Sulfur distribution in distillate cuts Thiols, sulfides &Benzo-thiophenes, Dibenzo-thiopenes, Total sulfur, Sample # disulfides,ppm Thiophenes, ppm ppm ppm ppm 10% cut 361 53 81 0 495 20% cut 433 23121 0 577 30% cut 530 30 157 4 721 30-100% cut 978 21 1031 25 2055 WholeJetA 824 25 674 17 1530

The above table A illustrates the characteristics of various cuts orsection of the bulk hydrocarbon fuel. The last row indicates the sulfurconcentrations of the bulk fuel defined as Whole JetA containing the1530 ppm sulfur level. Most of these sulfur species were identified aslight sulfurs such as thiols, sulfides, and disulfides (824 ppm) andheavier sulfurs such as benzo-thiophenes (674 ppm). The row identifiedas 10% cut represents the fraction of the fuel where only the lightestspecies are removed similar to a stage-one permeate, while the 20% cutrepresents the next fraction or the 10-20% cut which excludes speciesremoved in the 0-10% cut and left in the 20-100% cut. As indicated bythe values lighter fractions had less sulfur (495 to 721 ppm) than thebulk fuel (1530 ppm) indicating a relative sulfur in light fractionbetween 25% for the 10% cut and 50% for the 30% cut. Also the dataindicates that most of the heavy sulfur species, represented by theDibenzo-thiopenes, remained in the 30-100% cut which would be thestage-one retentate that would be returned to the primary tank.

The second distillation cut (0-20% by volume) having 577 ppm totalsulfur was preheated to 125° C. and passed through a stage-two membrane(SB4034.4) that was maintained at 125° C. and at 26″ Hg vacuum. Permeatefrom the second stage (at a 20% stage-cut) contained 1130 ppm totalsulfur and the retentate from stage two contained 500 ppm total sulfur.The total sulfur reduction after stage one is 62% and the total sulfurreduction after stage two is 67%. The gasoline range sulfur, which isdefined as sulfur in compounds boiling below 220° C. (428° Fahrenheit),is reduced from 200 ppm in stage one product to 120 ppm in the stage-tworetentate. This test indicates preliminary data that supports theeffectiveness of the concept, but additional selectivity will be neededfor most practical applications.

EXAMPLE 6 Flash Vaporizer-Fractional Distillation as Stage-OneSeparation and Polar Pervaporation Membrane as the Stage-Two Separationwith 9% Cut in Permeate Stage-One and 25% Cut in Stage-Two Separation

A Jet fuel sample was analyzed using gas chromatograph equipped withflame photometric detectors (PFPD). Table B below indicates the sulfurspecies for both the whole jet fuel and for the 9% cut obtained from thestage-one permeate. The sulfur species are shown in greater detail andhave been organized within the table with the lightest species at thetop of the table and the heavier species at the bottom of the table. Asubtotal is provided for the sulfur species typically found in thegasoline fuel fractions indicating the light sulfur species of interest.As indicated by the data the relative sulfur in light fraction wasapproximately 40% (611 ppm/1473 ppm). The 9% cut also illustrated theelimination of a greater fraction of the heavy sulfur species, thoseabove the gasoline range.

TABLE B The 9% cut sample from fractional distillation (1^(st)) stage)whole jet 9% cut Mercaptans 3 4 Thiophene 1 1 MethylThiophenes 2 3TetrahydroThiophene 1 1 C2-Thiophenes 3 13 C3-Thiophenes 23 67C4-Thiophenes 146 167 Thiophenol 3 5 MethylThiophenol 6 9 SubtotalGasolineRange S 188 270 BenzoThiophene 34 7 C1-Benzothiophenes 257 157C2-Benzothiophenes 362 109 C3-Benzothiophenes 488 68 C4+-Benzothiophenes141 0 Dibenzothiophenes 3 0 Total S, ppm by wt 1473 611

In stage two, the 9% cut distillate was preheated to 125° C. and fed tostage-two membrane maintained at 125° C. and 26″ Hg vacuum. The permeatefrom stage two (at a 26% stage cut) contained 710 ppm sulfur and theretentate contained 427 ppm sulfur, again indicating the sulfurselectivity of the stage-two pervaporation membrane. The gasoline-rangesulfur is removed selectively by the stage-two pervaporation processthrough the membrane, whereas the higher boiling sulfur compounds arenot reduced effectively. It is therefore preferable to get a lighterdistillate cut that eliminates more of the trace heavy sulfur species atthe stage-one separation process for more effective, overalldesulfurization process.

EXAMPLE 7 High Flux Pervaporation Membrane as the Stage-1A and Stage-1BSeparation

This example pertains particularly to FIG. 5 in which two condensers 370and 371 are utilized. The Jet-A fuel sample was sent through a high fluxmembrane at 120° C. and 26″ Hg vacuum functioning as a stage-1A process.The stage-1A permeate was condensed and sent to stage-1B high fluxmembrane stage to further reduce the heavy sulfur species. Here, furtherremoval of heavy, high boiling sulfur compounds were rejected in theretentate. A 10% stage-one cut was extracted as permeate and the resultsare provided in Table C1 below. The data indicates the relative sulfurin light fraction was under 15% (181 ppm versus 1473 ppm) illustratingthe enhanced performance when the stage-one separation is subdividedinto a stage-1A and stage-1B.

TABLE C High Flux membrane as Stage-one with Jet-A fuel whole jetpermeate Mercaptans 3 4 Thiophene 1 1 MethylThiophenes 2 3TetrahydroThiophene 1 1 C2-Thiophenes 3 13 C3-Thiophenes 23 40C4-Thiophenes 146 48 Thiophenol 3 4 MethylThiophenol 6 8 GasolineRange S188 122 BenzoThiophene 34 1 C1-Benzothiophenes 257 15 C2-Benzothiophenes362 43 C3-Benzothiophenes 488 C4+-Benzothiophenes 141 Dibenzothiophenes3 Total S, ppm by wt 1473 181

The stage-1B permeate was condensed and sent to the sulfur selectiveS-Brane membrane separator. The stage-two retentate is recovered as theclean auxiliary power fuel stream. The stage-1A and stage-1B retentatesare combined with the permeate from the stage-two and sent back to theprimary fuel tank.

Changes may be made in the above systems and processes without departingfrom the scope of the disclosure herein involved. It is thereforeintended that all matter contained in the above description, and asshown in the accompanying drawing, shall be interpreted as illustrative,and exemplary. It is not intended that the disclosure be limited to theillustrated embodiments.

1. A process for producing an auxiliary fuel stream containing a lowconcentration of sulfur compounds from a primary fuel stream, theprocess comprising: passing a portion of a primary fuel stream through afirst separation stage comprising isolating a stage-one permeate streamand a stage-one retentate stream from the portion of the primary fuelstream; and passing the stage-one permeate stream through a secondseparation stage comprising isolating a stage-two permeate stream and astage-two retentate stream from the stage-one permeate stream whereinthe stage-two retentate stream constitutes an auxiliary fuel streamcontaining low concentration of sulfur compounds, wherein the stage-oneretentate stream and said stage-two permeate stream are mixed andreturned to the primary fuel stream.
 2. The process of claim 1, whereinthe operation of isolating the stage-one permeate stream and thestage-one retentate stream from the primary fuel stream is performed bypassing the portion of the primary fuel stream through a stage-onepervaporation membrane at a predetermined stage-one pervaporationtemperature and at a predetermined stage-one pervaporation vacuum. 3.The process according to claim 1 wherein the operation of isolating astage-two permeate stream and a stage-two retentate stream from thestage-one permeate stream, is performed by passing the stage-onepermeate stream through a stage-two pervaporation membrane at apredetermined stage-two pervaporation temperature and at a predeterminedstage-two pervaporation vacuum.
 4. The process according to claim 2wherein isolating a stage-one permeate stream and a stage-one retentatestream from the primary fuel stream is performed by a high-throughput,low selectivity stage-one pervaporation membrane at the predeterminedstage-one pervaporation temperature and the predetermined stage-onepervaporation vacuum.
 5. The process according to claim 3 whereinisolating a stage-two permeate stream and a stage-two retentate streamfrom the stage-one permeate stream is performed by passing the stage-onepermeate stream through a high selectivity stage-two pervaporationmembrane at the predetermined stage-two pervaporation temperature andthe predetermined stage-two pervaporation vacuum.
 6. The processaccording to claim 4, wherein the vacuum in stage-one is created byrecirculating cooled stage-one permeate stream at a high flow rate. 7.The process according to claim 5, wherein the vacuum in stage two iscreated by recirculating a pressurized stage-one retentate.
 8. A processfor separating a fuel stream containing a low concentration of sulfurcompounds from a primary fuel stream, the process comprising: isolatinga stage-one permeate stream and a stage-one retentate stream from theprimary fuel stream; evaporating the stage-one permeate stream at avacuum; and isolating a stage-two permeate stream and a stage-tworetentate stream from the stage-one permeate stream wherein thestage-one retentate stream and said stage-two permeate stream are mixedand returned to the primary fuel stream, the stage-two retentate streamcomprising a fuel stream containing low concentrations of sulfurcompounds.
 9. The process according to claim 8 further comprisingevaporating the stage-two permeate stream at a vacuum.
 10. The processaccording to any one of claims 1 to 7, wherein the stage-one separationprocess comprises performing a two stage condensation process.
 11. Asystem for separating a fuel stream containing low concentrations ofsulfur compounds from a primary fuel stream, the system comprising: afuel supply providing a primary fuel stream; a stage-one separator; afirst supply line connecting a portion of the primary fuel stream to thestage-one separator, the stage-one separator separating the portion ofthe primary fuel stream into a stage-one permeate stream and a stage-oneretentate stream; a stage-two separator; a second supply line connectingthe stage-one separator permeate stream to the stage-two separator;wherein the stage-two separator isolates the stage-one separatorpermeate stream into a stage-two permeate stream and a stage-tworetentate stream, and wherein the stage-two retentate stream contains alow concentration of sulfur compounds, and wherein the stage-oneretentate stream and said stage-two permeate stream are mixed andreturned to the primary fuel stream.
 12. The system of claim 11 furthercomprising at least one heat exchanger in the first supply line heatingthe portion of the primary fuel stream to between 100° to 250° C. 13.The system of claim 11 further comprising at least one heat exchanger inthe second supply line, the heating exchanger heating the stage-onepermeate to between 100° to 250° C.
 14. The system of claim 11 furthercomprising an eductor used to create a vacuum using the stage-oneretentate.
 15. The system of claim 14 wherein the eductor provides thevacuum for the stage-one permeate stream, by using the condensedstage-one permeate stream as its motive fluid.
 16. The system accordingto claim 11 wherein the primary fuel is heated by heat from an engine.17. The system according to any one of claims 11 to 16 furthercomprising a third stage polishing filter connected to the stage-tworetentate stream to further reduce sulfur concentrations.
 18. The systemaccording to any one of claims 12 to 13, wherein the heat exchanger is arecuperative heat exchanger.